1. Field of the Invention
This invention relates to a gas current classifier (an air classifier) for classifying powder utilizing the Coanda effect. More particularly, the present invention relates to a gas current classifier for classifying powder to obtain particles having a given particle size utilizing the Coanda effect and the differences in inertia force and centrifugal force according to the particle size of each particle of the powder while the powder is carried on gas streams, so that a powder in which particles of 20 .mu.m or smaller diameter are 50% by number or more can be obtained efficiently.
This invention also relates to a process for producing a toner by means of a gas current classifier for classifying a colored resin powder utilizing the Coanda effect. More particularly, the present invention relates to a process for producing a toner for developing electrostatic images, by classifying colored resin powder to collect particles having a given particle size based on the Coanda effect and the differences in inertia force and centrifugal force according to the particle size of each particle of the powder while the powder is carried on a gas stream, so that a colored resin powder in which particles of 20 .mu.m or smaller diameter are 50% by number or more can be obtained efficiently.
2. Related Background Art
For powder classification, various gas current classifiers have been proposed. There are classifiers having rotating blades and those having no moving parts. The classifiers having no moving parts include fixed-wall centrifugal classifiers and inertial classifiers. In classifiers utilizing inertia force, Elbow Jet classifier disclosed in Loffier, F. and K. Maly, Symposium on Powder Technology D2 (1981) and commercially available from Nittetsu Kogyo, and a classifier disclosed in Okuda, S. and Yasukuni, J., Proceedings of International Symposium on Powder Technology `81, 771 (1981) were contrived as an inertial classifier which can carry out classification in a fine-powder range.
In such a gas current classifier, as shown in FIGS. 9 and 10, the material powder is jetted into the classification zone of a classifying chamber 32 at a high speed with a gas stream, from a material feeding nozzle 16 having an orifice to the classification zone. A gas stream is introduced in the classifying chamber to cross the gas stream emitted from the material feed nozzle 16 so that by the action of centrifugal force produced by the curved gas stream along the Coanda block 26 provided in the chamber the powder is classified into three fractions of coarse powder, medium powder and fine powder and separated by means of classifying edges 117 and 118 each having a tapered tip.
In such a conventional classifier 101, however, as shown in FIG. 12, the material powder fed from a material receiving opening 40 into the material feed nozzle 16, flows in the material feed nozzle 16, showing a tendency to flow along the wall of the nozzle. Here, in the material feed nozzle 16, the material powder fed downward tends to be gravity-classified, so that light fine powder tends to be enriched in the upper stream of the path and heavy coarse powder in the lower stream in the path. Thus, as shown in FIG. 13, the coarse particles in the lower stream disturb the movement of the fine particles in the upper stream, and there has been a limit in the improvement of classification precision. Moreover, with a powder containing coarse particles with particle diameters of 20 .mu.m or larger much, the precision tends to decrease.
Especially when the classification of the material powder is carried out in the production process of a toner to be used in image forming apparatus such as copying machines and electrophotographic printers, the classified fractions of particles are required to have sharp particle size distributions, and it is also important that the cost of the classification is low and the efficiency is high as well as classification precision.
From such points of view, required is a gas current classifier that can stably and efficiently classify powder, in particular, colored fine resin powder such as a toner in a good precision.